WO2022004415A1 - Method for producing optical fiber base material, and optical fiber base material - Google Patents

Abstract

This method for producing an optical fiber base material includes: adding an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe comprising a silica-based glass; reducing the diameter of the glass pipe following the addition; etching the inner surface in a series of regions in the longitudinal direction of the glass pipe following the diameter reduction; and collapsing the glass pipe following the etching. At least one of the addition, the diameter reduction, the etching and the collapsing includes locally etching the inner surface a region that is shorter than the series of regions in the glass pipe.

Description

Manufacturing method of optical fiber base material and optical fiber base material

This disclosure relates to a method for manufacturing an optical fiber base material and an optical fiber base material. This application claims priority based on Japanese Application No. 2020-115688 filed on July 3, 2020, and incorporates all the contents described in the Japanese application.

If the core made of silica-based glass contains an alkali metal element or an alkaline earth metal element, the viscosity of the core is reduced when the optical fiber base material is drawn to manufacture the optical fiber, and the glass is regenerated. The sequence is promoted. Therefore, the transmission loss due to the ray scattering of the optical fiber is reduced. As a result, the transmission loss can be reduced.

Patent Document 1, Patent Document 2, and Patent Document 3 describe a method of adding a metal element or an alkaline earth metal element to the core portion of an optical fiber base material by a diffusion method.

International Publication No. 2004/20357 International Publication No. 2005/021344 International Publication No. 2013/111470

The method for producing the optical fiber base material of the present disclosure is to add an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe made of silica-based glass, and to reduce the diameter of the glass pipe after the addition. It includes etching the inner surface of a series of longitudinal sections of the glass pipe after diameter and collapsing the glass pipe after etching. At least one of adding, reducing the diameter, etching, and collapsing involves locally etching the inner surface of a section of the glass pipe that is shorter than a series of sections.

The method for producing the optical fiber base material of the present disclosure is to add an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe made of silica-based glass, and to reduce the diameter of the glass pipe after the addition. Etching and etching the inner surface of a series of longitudinal sections of the glass pipe after diameter, and collapsing the glass pipe after etching, and between adding and reducing the diameter. This includes locally etching a section of the inner surface of the glass pipe that is shorter than a series of sections, and at least one of between etching and collapsing.

The optical fiber base material of the present disclosure is made of silica-based glass and includes a core portion containing an alkali metal element or an alkaline earth metal element, and the concentration of the alkali metal element or the alkaline earth metal element in the core portion is in the longitudinal direction. It is different for some parts and other parts.

The optical fiber base material of the present disclosure is made of silica-based glass and has a core portion containing an alkali metal element or an alkaline earth metal element, and the core portion contains bromine in a part or all in the longitudinal direction.

It is a flowchart which shows the manufacturing method of the optical fiber which concerns on embodiment. It is a figure explaining the addition process. It is sectional drawing of the optical fiber base material which concerns on embodiment.

[Problems to be solved by this disclosure]
In the methods described in Patent Document 1, Patent Document 2, and Patent Document 3, crystallization of glass occurs after starting to add an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe, and the portion thereof. May become a defective part. In addition, the fine particles may be scattered from the crystal and the defective portion may spread to other parts.

It is an object of the present disclosure to provide a method for manufacturing an optical fiber base material and an optical fiber base material capable of improving productivity while suppressing transmission loss.

[Effect of this disclosure]
According to the present disclosure, it is possible to provide a method for manufacturing an optical fiber base material and an optical fiber base material capable of improving productivity while suppressing transmission loss.

[Explanation of Embodiments of the present disclosure]
First, embodiments of the present disclosure will be listed and described. In the method for producing an optical fiber base material according to an embodiment of the present disclosure, an alkali metal element or an alkaline earth metal element is added to the inner surface of a glass pipe made of silica-based glass, and the diameter of the glass pipe is reduced after the addition. This includes etching the inner surface of a series of longitudinal sections of the glass pipe after diameter reduction and collapsing the glass pipe after etching. At least one of adding, reducing the diameter, etching, and collapsing involves locally etching the inner surface of a section of the glass pipe that is shorter than a series of sections. Here, "local etching" is shorter than the fixed length range within or outside the fixed length range in which the heat source moves during addition, reduction, etching, and collapse. It means etching limited to one or more sections. Even in a non-local etching process, the amount of glass to be scraped is not constant due to fluctuations in the moving speed of the heat source and changes in the flow of the etching gas, and there are minute fluctuations in the amount of glass to be scraped. However, local etching is performed by intentionally changing conditions such as heat source temperature, etching gas flow rate, and heat source transfer speed during addition, diameter reduction, etching, and collapse, or addition, diameter reduction. , Etching, added separately during the collapse process.

In the method for producing an optical fiber base material according to another embodiment of the present disclosure, an alkali metal element or an alkaline earth metal element is added to the inner surface of a glass pipe made of silica-based glass, and the glass pipe is shrunk after the addition. Shrinking between scaling, etching the inner surface of a series of longitudinal sections of the glass pipe after shrinking, collapsing the glass pipe after etching, adding and shrinking. Locally etching the inner surface of a section of a glass pipe that is shorter than a series of sections in at least one of between calibrating and etching, and between etching and collapsing. And, including.

In any of the methods for manufacturing an optical fiber base material according to any embodiment, defective parts such as crystallization generated on the inner surface of the glass pipe can be removed. Therefore, it is possible to prevent the defective portion from expanding due to the enlargement of the generated crystal and the fine particles scattered from the crystal causing crystals to be generated at other locations and the number of defective portions to increase. As a result, productivity can be improved while suppressing transmission loss by adding an alkali metal element or an alkaline earth metal element.

Local etching may be performed on one or more points in a series of sections. In this case, defective parts such as crystallization generated in the part to be a product can be removed.

Local etching may be performed on one or more locations other than a series of sections. In this case, defective parts such as crystallization generated in parts other than the parts to be made into products can be removed.

The optical fiber base material according to the embodiment of the present disclosure is made of silica-based glass, includes a core portion containing an alkali metal element or an alkaline earth metal element, and the concentration of the alkali metal element or the alkaline earth metal element in the core portion. Is different for a part in the longitudinal direction and a part other than a part.

In this optical fiber base material, the concentration of alkali metal element or alkaline earth metal element is locally low due to the effect of local etching. However, the difference in transmission loss at a wavelength of 1550 nm is small between the optical fiber derived from the locally etched portion and the optical fiber derived from the locally etched portion.

The optical fiber base material according to the embodiment of the present disclosure is made of silica-based glass, includes a core portion containing an alkali metal element or an alkaline earth metal element, and the core portion contains bromine in a part or all in the longitudinal direction. Includes.

In this optical fiber base material, by locally etching, the glass viscosity can be lowered due to the effect of the addition of bromine, and the transmission loss becomes lower than that in the case where the bromine is not added.

[Details of Embodiments of the present disclosure]
The manufacturing method of the optical fiber base material and the specific example of the optical fiber base material of the present disclosure will be described below with reference to the drawings. It should be noted that the present disclosure is not limited to these examples, but is shown by the scope of claims and is intended to include all modifications within the meaning and scope equivalent to the scope of claims. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate description is omitted.

FIG. 1 is a flowchart illustrating a method for manufacturing an optical fiber according to the present embodiment. The following description also describes an example of specific conditions. The optical fiber includes a preparation process S1, an addition process S2, a diameter reduction process S3, an etching process S4, a collapse process S5, a drawing grinding process S6, a rod-in collapse process S7, an OVD (Outside Vapor Deposition) process S8, and a drawing process S9. Manufactured in order. The optical fiber base material 10 (see FIG. 3) is manufactured through a preparation step S1, an addition step S2, a diameter reduction step S3, an etching step S4, a collapse step S5, a draw grinding step S6, a rod in collapse step S7, and an OVD step S8. Will be done. That is, the method for manufacturing the optical fiber base material according to the present embodiment includes the preparation step S1, the addition step S2, the diameter reduction step S3, the etching step S4, the collapse step S5, the draw grinding step S6, the rod in collapse step S7, and the OVD step. Including S8.

The preparation step S1 is a step of preparing a glass pipe to diffuse the alkali metal group as a dopant. Here, the alkali metal group is a general term for alkali metal elements and alkaline earth metal elements. The glass pipe is made of silica (quartz) glass. The silica-based glass rod that is the basis of this glass pipe is manufactured by, for example, the VAD (Vapor phase axial depression) method. A pipe is manufactured by making a hole in the cylinder and then stretching it. The silica-based glass rod that forms the basis of the glass pipe contains a certain concentration of chlorine and fluorine. The mass fraction of other dopants and impurities is 10 ppm or less. Here, the mass fraction is the ratio of the mass of the element of interest to the total mass, and is expressed as (mass of the element of interest) / (total mass). Hereinafter, the mass fraction is referred to as "concentration".

The outer diameter (2d) of the glass pipe is 30 mm or more and 50 mm or less. The inner diameter (2i) of the glass pipe is 10 mm or more and 30 mm or less. The glass pipe contains chlorine having an average concentration of 0 or more and 2500 ppm or less, and fluorine having an average concentration of 1000 ppm or more and 5000 ppm or less. The concentration of dopants and impurities other than chlorine and fluorine in the glass pipe is 10 ppm or less. Here, the average concentration is, for example, the concentration represented by the following formula in the case of the average chlorine concentration.

In the above formula, Cl (r) represents the local chlorine concentration at the position of the radius r. i represents the inner radius of the glass pipe. d represents the outer radius of the glass pipe. In the case of the glass rod as well, the average concentration can be expressed by the above formula by setting i to 0 and d to the outer radius of the glass rod. The local concentration is measured by an electron probe microanalyzer (EPMA) as a concentration at each position along a straight line passing through a central position on an end face of a glass pipe and a glass rod. The conditions for measurement by EPMA are, for example, an acceleration voltage of 20 kV, a probe beam diameter of 1 μm or less, and a measurement interval of 100 nm or less.

The addition step S2 is a step of adding an alkali metal group to the inner surface of a glass pipe made of silica-based glass. When the potassium (K) element is added as a dopant of the alkali metal group, for example, potassium bromide (KBr) of 6 g or more and 20 g or less is used as a raw material. Depending on the type of alkali metal group to be added, one or more of KBr, potassium iodide (KI), rubidium bromide (RbBr), rubidium iodide (RbI) and the like may be used as a raw material.

FIG. 2 is a diagram illustrating an addition process. As shown in FIG. 2, a handling glass pipe 5 arranged in the electric furnace 2 is connected to one end of the glass pipe 1. A part of the handling glass pipe 5 is used as a raw material reservoir, and the raw material 3 is installed. A part of the glass pipe 1 may be used as a raw material reservoir. An oxyhydrogen burner 4 is arranged outside the glass pipe 1. The electric furnace 2 is an external heat source for heating the raw material 3, and the oxyhydrogen burner 4 is an external heat source for heating the glass pipe 1. Instead of the oxyhydrogen burner 4, an induction furnace, a resistance furnace, or the like may be used.

The raw material 3 is heated to a temperature of 700 ° C. or higher and 850 ° C. or lower by the electric furnace 2 to generate raw material steam. The glass pipe 1 is heated from the outside by the oxyhydrogen burner 4 while introducing the generated raw material vapor into the inside of the glass pipe 1 together with the carrier gas composed of oxygen. The flow rate of the carrier gas is 1 SLM (volume in the standard state (25 ° C., 100 kPa) of the gas flowing per minute) or more and 3 SLM or less. In the addition step S2, the glass pipe 1 is heated by moving the oxyhydrogen burner 4 (external heat source) along the longitudinal direction of the glass pipe 1. The heating of the glass pipe 1 is performed by traversing the oxyhydrogen burner 4 at a speed of 30 mm / min or more and 60 mm / min or less so that the temperature of the outer surface of the glass pipe 1 becomes 1400 ° C. or higher and 2000 ° C. or lower for a total of 8 turns. It will be done in 15 turns or less. As a result, the alkali metal group is diffusely added to the inner surface of the glass pipe 1.

The diameter reduction step S3 is a step of reducing the diameter of the glass pipe to which the alkali metal group is added after the addition step S2. At this time, the glass pipe is heated from the outside by an external heat source while oxygen is flowing inside the glass pipe at 0.5 SLM or more and 1.0 SLM or less. In the diameter reduction step S3, the glass pipe is heated by moving the external heat source along the longitudinal direction of the glass pipe. The heating of the glass pipe is performed in a total of 6 turns or more and 10 turns or less by traversing an external heat source so that the outer surface of the glass pipe becomes 1300 ° C. or higher and 2000 ° C. or lower. As a result, the glass pipe is reduced in diameter until the inner diameter is 3 mm or more and 5 mm or less.

The etching step S4 is a step of etching the inner surface of a series of longitudinal sections of the glass pipe after the diameter reduction step S3. At this time, _{while introducing a mixed gas of SF 6} (0.2 SLM or more and 0.4 SLM or less) and chlorine (0.5 SLM or more and 1.0 SLM or less) into the inside of the glass pipe, the glass pipe is heated from the outside by an external heat source. Gas phase etching is performed. By doing so, it is possible to scrape the inner surface of the pipe containing impurities added together with the target dopant at a high concentration, and it is possible to remove these impurities. In the etching step S4, the glass pipe is heated by moving the external heat source along the longitudinal direction of the glass pipe. The heating of the glass pipe is performed in a total of 1 turn or more and 5 turns or less by traversing an external heat source so that the outer surface of the glass pipe becomes 1300 ° C. or higher and 2000 ° C. or lower.

The collapse step S5 is a step of collapsing the glass pipe after the etching step S4. A mixed gas of oxygen (0.1 SLM or more and 0.5 SLM or less) and He (0.5 SLM or more and 1.0 SLM or less) is introduced into the glass pipe, and the absolute pressure in the glass pipe is reduced to 97 kPa or less while reducing the surface surface. The temperature is set to 2000 or more and 2300 ° C. or less, and the glass pipe is closed. As a result, the glass pipe is solidified, and a glass rod (medium substance) having a diameter (outer diameter) of 20 mm or more and 40 mm or less can be obtained.

In the draw grinding step S6, the glass rod is stretched to have a diameter of 20 mm or more and 25 mm or less, and the outer peripheral portion of the glass rod is further ground to have a diameter of 15 mm or more and 20 mm or less. The glass rod (core rod) thus obtained becomes the core portion 11 (see FIG. 3) of the optical fiber base material 10. A core layer containing no alkali metal group may be provided around the core portion 11 by a known method such as an OVD method or a collapse method.

In the rod incolapse step S7, the first clad portion 12 (see FIG. 3) is provided on the outside of the core portion 11. At this time, a rod-in-collapsing method is used in which the core portion 11 is inserted inside a glass pipe of silica-based glass containing fluorine, and both are heated and integrated by an external heat source. As a result of the addition of the first clad portion 12 by this rod incollapsing method, the water content of the core portion 11 and the first clad portion 12 in the vicinity thereof can be suppressed to be sufficiently low.

In the OVD step S8, a rod in which the core portion 11 and the first clad portion 12 are integrated is stretched to have a predetermined diameter, and then a second clad portion 13 (see FIG. 3) containing fluorine is attached to the outside of the rod. The optical fiber base material 10 is manufactured by synthesizing by the OVD method.

In the drawing step S9, an optical fiber can be obtained by drawing the optical fiber base material 10. The drawing speed is 800 m / min or more and 2300 m / min or less, and the drawing tension is, for example, 0.5 N.

As described above, in the addition step S2, the alkali metal group is added to the inner surface of the glass pipe. The glass pipe contains chlorine and fluorine for the suppression of glass defects and the adjustment of the refractive index. Therefore, chlorides and fluorides of the alkali metal group may be generated, and the surrounding silica glass may crystallize with them as nuclei. If the crystallization of silica glass is left unattended and the addition step S2 is advanced, the size of the defective portion increases due to the increase in size of the crystals, and the yield decreases. In addition, the defective portion (crystal portion) may generate crystals having fine particles as nuclei in other portions by scattering fine particles such as glass. As the process of forming the defective portion, there are various cases where not only the case caused by chloride and fluoride but also the case where a minute foreign substance flowing from the upstream of the glass pipe becomes a nucleus.

The above-mentioned crystallization may occur in any step from the addition step S2 to the collapse step S5. The problem is that the defective part expands due to the enlargement of the generated crystal, and the fine particles scattered from the crystal cause crystals to be generated at other parts and the number of defective parts increases.

Therefore, in the method for manufacturing an optical fiber base material according to the present embodiment, when a defect such as crystallization occurs, in order to remove the defective portion such as the crystal portion, the section of the glass pipe shorter than a series of sections is included. Additional local etching is performed only on the surface. The local etching is performed in at least one of the addition step S2, the diameter reduction step S3, the etching step S4, and the collapse step S5. Local etching is performed in at least one of the addition step S2 and the diameter reduction step S3, the diameter reduction step S3 and the etching step S4, and the etching step S4 and the collapse step S5. You may.

By performing additional local etching, it is possible to prevent the defective part, which is an abnormal part, from expanding, and the defective part from being generated at other parts to increase the number of defective parts. Most of the abnormal parts scatter light unlike the normal parts. Therefore, the abnormal portion can be identified by irradiating the light and detecting the scattered light. Local etching may be performed until no light scattering due to the abnormal portion is observed. However, even if the abnormal part is not completely removed, it may be possible to suppress the scattering to other parts by reducing the size of the abnormal part.

At the location where local etching was performed, the alkali metal group added in the addition step S2 may be removed by etching, and the concentration of the alkali metal group may change locally. As a result, there may be a difference between the characteristics of the transmission loss at the locally etched portion and the characteristics of the transmission loss at the other portion. However, even in this case, in the optical fiber after drawing, only the portion having different characteristics becomes a defective portion, so that the reduction in yield can be minimized. Therefore, it is possible to improve the productivity while suppressing the transmission loss by adding the alkali metal group.

Local etching is performed on one or more locations within the moving range of an external heat source such as an oxyhydrogen burner in the addition step S2, the diameter reduction step S3, and the etching step S4. Local etching is effective not only for the moving range of the external heat source but also for the abnormal part such as a crystal generated outside the moving range of the external heat source. This is because the fine particles may be scattered from the abnormal portion generated outside the moving range of the external heat source, causing crystals to be generated in other places. Therefore, the local etching may be performed on one or more locations outside the moving range of the external heat source such as the oxyhydrogen burner in the addition step S2, the diameter reduction step S3, and the etching step S4.

The local etching is a gas phase etching, in which _{a mixed gas of SF 6} (0.1 SLM or more and 0.4 SLM or less) and chlorine (0.5 SLM or more and 1.0 SLM or less) is introduced into the glass pipe while being introduced to the outside. This is done by heating the glass pipe with a heat source. The local etching may be performed with the external heat source fixed at the location of the abnormal portion, or may be performed while moving the external heat source. As the etching gas, _{a mixed gas of SF 6} and bromine may be used instead of the mixed gas of _{SF 6 and chlorine.} When local etching is performed in the addition step S2, the diameter reduction step S3, the etching step S4, or the collapse step S5, the gas flowing in the step is temporarily stopped and the above-mentioned flow rate of SF ₆ and chlorine is flowed. Local etching may be performed. Local etching is performed with the surface temperature of the pipe set to 1000 ° C. or higher and 1500 ° C. or lower by an external heat source.

FIG. 3 is a cross-sectional view of the optical fiber base material according to the present embodiment. As shown in FIG. 3, the optical fiber base material 10 includes a core portion 11, a first clad portion 12, and a second clad portion 13. The core portion 11 is made of silica-based glass. The core portion 11 contains an alkali metal group, chlorine, and fluorine. The average concentration of the alkali metal group in the core portion 11 is 10 ppm or more and 100 ppm or less. The average concentration of chlorine in the core portion 11 is 50 ppm or more and 2000 ppm or less. The average concentration of fluorine in the core portion 11 is 2000 ppm or more and 3500 ppm or less. Due to the additional local etching performed in the manufacturing process, the concentration of the alkali metal group in the core portion 11 is different from the portion other than the portion in the longitudinal direction. In the core portion 11, the concentration of the alkali metal group in the portion where the local etching is performed is lower than the concentration of the alkali metal group in the other portion. When the local etching is _{performed using a mixed gas of SF 6} and bromine, the core portion 11 contains bromine in part or all of the longitudinal direction. Bromine is contained in the center of the core portion 11.

The first clad portion 12 is provided on the outside of the core portion 11 and surrounds the core portion 11. The first clad portion 12 is made of silica-based glass. The first clad portion 12 contains fluorine. The difference in the refractive index standardized by the refractive index of the pure silica glass between the core portion 11 and the first clad portion 12 is about 0.34% at the maximum.

The second clad portion 13 is provided on the outside of the first clad portion 12 and surrounds the first clad portion 12. The second clad portion 13 is made of silica-based glass. The second clad portion 13 contains fluorine.

In the following, a prototype example will be described using Tables 1 to 5.

Table 1 is a table summarizing the specifications and evaluation results of Prototype Example 1 to Prototype Example 6. “Cl concentration [ppm]” in Table 1 indicates the average chlorine concentration contained in the glass rod after the collapse step S5 is completed. "F concentration [ppm]" indicates the average fluorine concentration contained in the glass rod after the collapse step S5 is completed. "K concentration [ppm]" indicates the average potassium concentration contained in the glass rod after the collapse step S5 is completed. The “abnormal number of parts” indicates the number of abnormal parts such as crystals remaining on the glass rod after the collapse step S5 is completed. "Local etching" indicates the presence or absence of local etching and the etching gas used when there is. “Raw material” indicates a raw material used in the addition step S2.

In Prototype 1 to Prototype 6, 15 g of KBr was used as a raw material in the addition step S2. In Prototype 1 to Prototype 4, local etching was not performed. From the evaluation results of Prototype Example 1 to Prototype Example 4, it was found that the higher the total value of the average chlorine concentration and the average fluorine concentration in the glass pipe, the larger the number of abnormal parts when local etching was not performed. ..

In Prototype 5 and Prototype 6, local etching was performed using the etching gas as a mixed gas of _{SF 6 and chlorine.} Local etching may be performed immediately when an abnormality occurs, or it may be performed after one or more steps. In Prototype Example 5, local etching was performed on the abnormal portion within the moving range of the external heat source in the addition step S2, the diameter reduction step S3, and the etching step S4. In Prototype Example 5, the number of abnormal copies could be reduced to 3. In Prototype Example 6, local etching was performed not only within the moving range of the external heat source in the addition step S2, the diameter reducing step S3, and the etching step S4, but also on the abnormal portion generated outside the moving range. In Prototype Example 6, the number of abnormal copies could be reduced to zero.

An optical fiber was obtained by drawing an optical fiber base material manufactured using the glass rods of Prototype Example 5 and Prototype Example 6. Also in Prototype 5 and Prototype 6 in which local etching was performed, an optical fiber having a transmission loss at a wavelength of 1550 nm of 0.148 dB / km or more and 0.150 dB / km or less was obtained over the entire length of the optical fiber base material. rice field. From this, it was found that there was no significant difference in transmission loss between the portion where the local etching was performed and the portion where the local etching was not performed. This is presumed to be due to the following reasons. That is, in the portion where the local etching is performed, the potassium concentration is reduced, so that the Rayri scattering loss is increased. However, since the inner surface of the glass pipe is smoothed by the extra heating, the loss due to the structural irregularity in the subsequent collapse step S5 is reduced.

Table 2 is a table summarizing the specifications and evaluation results of Prototype Example 7 to Prototype Example 12. “Rb concentration [ppm]” in Table 2 indicates the average rubidium concentration contained in the glass rod after the collapse step S5 is completed. The other items are the same as those in Table 1. The average rubidium concentration is obtained by the above formula after measuring the local concentration using the above-mentioned EPMA, similarly to the average chlorine concentration and the average fluorine concentration.

In Prototype 7 to 12, 15 g of RbBr was used as a raw material in the addition step S2. In Prototype Example 7 to Prototype Example 10, local etching was not performed. From the evaluation results of Prototype Example 7 to Prototype Example 10, it was found that the higher the total value of the average chlorine concentration and the average fluorine concentration in the glass pipe, the larger the number of abnormal parts when local etching was not performed. ..

In Prototype 11 and Prototype 12, local etching was performed in the same manner as in Prototype 5 and Prototype 6. In Prototype Example 11, local etching was performed on the abnormal portion within the moving range of the external heat source in the addition step S2 and the diameter reduction step S3. In Prototype Example 11, the number of abnormal copies could be reduced to 2. In Prototype Example 12, local etching was performed not only within the moving range of the external heat source in the addition step S2, the diameter reducing step S3, and the etching step S4, but also on the abnormal portion generated outside the moving range. In Prototype Example 12, the number of abnormal copies could be reduced to 1.

From the evaluation results of Prototype Example 7 to Prototype Example 12, it was found that the final number of abnormal copies can be reduced by local etching not only for potassium but also for rubidium.

Table 3 is a table summarizing the specifications and evaluation results of Prototype Example 13 to Prototype Example 18. Each item in Table 3 is the same as in Table 1.

In Prototype 13 to 18, 15 g of KI was used as a raw material in the addition step S2. In Prototype 13 to 16, local etching was not performed. From the evaluation results of Prototype Example 13 to Prototype Example 16, it was found that the higher the total value of the average chlorine concentration and the average fluorine concentration in the glass pipe, the larger the number of abnormal parts when local etching was not performed. ..

In Prototype 17 and Prototype 18, local etching was performed in the same manner as in Prototype 5 and Prototype 6. In Prototype Example 17, local etching was performed on the abnormal portion within the moving range of the external heat source in the addition step S2 and the diameter reduction step S3. In Prototype Example 17, the number of abnormal copies could be reduced to 3. In Prototype Example 18, local etching was performed not only within the moving range of the external heat source in the addition step S2, the diameter reducing step S3, and the etching step S4, but also on the abnormal portion generated outside the moving range. In Prototype Example 18, the number of abnormal copies could be reduced to 1.

From the evaluation results of Prototype Example 13 to Prototype Example 18, it was found that the final number of abnormal parts can be reduced by local etching not only for bromide but also for iodide.

Table 4 is a table summarizing the specifications and evaluation results of Prototype Example 19 to Prototype Example 24. “Rb concentration [ppm]” in Table 4 indicates the average rubidium concentration contained in the glass rod after the collapse step S5 is completed. The other items are the same as those in Table 1.

In Prototype 19 to 24, two types of KBr and RbBr were used as raw materials in the addition step S2, and the total mass was unified to 15 g. In Prototype 19 to 22, no local etching was performed. From the evaluation results of Prototype Example 19 to Prototype Example 22, it was found that the higher the total value of the average chlorine concentration and the average fluorine concentration in the glass pipe, the larger the number of abnormal parts when local etching was not performed. ..

In Prototype 23 and Prototype 24, local etching was performed in the same manner as in Prototype 5 and Prototype 6. In Prototype Example 23, local etching was performed on the abnormal portion within the moving range of the external heat source in the addition step S2 and the diameter reduction step S3. In Prototype Example 23, the number of abnormal copies could be reduced to 2. In Prototype Example 24, local etching was performed not only within the moving range of the external heat source in the addition step S2, the diameter reducing step S3, and the etching step S4, but also on the abnormal portion generated outside the moving range. In Prototype Example 24, the number of abnormal copies could be reduced to 1.

Table 5 is a table summarizing the specifications and evaluation results of the prototype 25 to the prototype 30. “Br concentration [ppm]” in Table 5 indicates the bromine concentration detected from the center of the glass rod (medium substance) after the collapse step S5 is completed. The other items are the same as those in Table 1. The bromine concentration can be determined using the above-mentioned EPMA.

In Prototype Example 25 to Prototype 28, 15 g of KBr was used as a raw material in the addition step S2. In Prototype Example 25 to Prototype 28, local etching was not performed. In Prototype 29 and Prototype 30, local etching was performed in the same manner as in Prototype 5 and Prototype 6 except _{that the etching gas was a mixed gas of SF 6 and bromine.} In Prototype Example 29, local etching was performed on the abnormal portion within the moving range of the external heat source in the addition step S2 and the diameter reduction step S3. In Prototype Example 29, the number of abnormal copies could be reduced to 1. In Prototype Example 30, local etching was performed not only within the moving range of the external heat source in the addition step S2, the diameter reducing step S3, and the etching step S4, but also on the abnormal portion generated outside the moving range. In Prototype Example 30, the number of abnormal copies could be reduced to zero.

From the evaluation results of Prototype Example 25 to Prototype Example 30, it is considered that it is important to remove the abnormal part without depending on the etching gas type in order to reduce the final number of abnormal parts.

An optical fiber was obtained by drawing an optical fiber base material manufactured using the glass rods of Prototype Example 29 and Prototype Example 30. In the portion where the local etching was not performed, the transmission loss at a wavelength of 1550 nm was 0.148 dB / km or more and 0.149 dB / km or less. On the other hand, in the portion where the local etching was performed, the transmission loss at a wavelength of 1550 nm was 0.147 dB / km. In this way, bromine is added because the result is that the transmission loss of the locally etched part is lower than the transmission loss of the locally etched part. It is considered that this is because the glass viscosity is lowered and the ray scattering loss is lowered.

1 ... Glass pipe 2 ... Electric furnace 3 ... Raw material 4 ... Oxyhydrogen burner (external heat source)
5 ... Handling glass pipe 10 ... Optical fiber base material 11 ... Core portion 12 ... First clad portion 13 ... Second clad portion

Claims (18)

1. Adding an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe made of silica-based glass,
   To reduce the diameter of the glass pipe after the addition,
   After the diameter reduction, etching the inner surface of a series of longitudinal sections of the glass pipe,
   Containing, including collapsing the glass pipe after the etching.
   At least one of the addition, the diameter reduction, the etching, and the collapse locally etches the inner surface of a section of the glass pipe that is shorter than the series of sections. Including to do,
   Manufacturing method of optical fiber base material.
2. Adding an alkali metal element or an alkaline earth metal element to the inner surface of a glass pipe made of silica-based glass,
   To reduce the diameter of the glass pipe after the addition,
   After the diameter reduction, etching the inner surface of a series of longitudinal sections of the glass pipe,
   Collapsing the glass pipe after the etching and
   The glass in at least one of the addition and the diameter reduction, the diameter reduction and the etching, and the etching and the collapse. Locally etching the inner surface of a section of the pipe that is shorter than the series of sections.
   Manufacturing method of optical fiber base material.
3. The local etching is performed on one or more points in the series of sections.
   The method for producing an optical fiber base material according to claim 1 or 2.
4. The local etching is performed on one or more places other than the series of sections.
   The method for manufacturing an optical fiber base material according to any one of claims 1 to 3.
5. The glass pipe contains chlorine and fluorine,
   The method for manufacturing an optical fiber base material according to any one of claims 1 to 4.
6. In the local etching, _{gas phase etching using a mixed gas of SF 6} and a mixed gas of chlorine is performed.
   The method for manufacturing an optical fiber base material according to any one of claims 1 to 5.
7. In the local etching, gas phase etching using a mixed gas of _{SF 6 and bromine is performed.}
   The method for manufacturing an optical fiber base material according to any one of claims 1 to 6.
8. The local etching includes identifying an abnormal portion by irradiating the glass pipe with light and detecting scattered light.
   The method for manufacturing an optical fiber base material according to any one of claims 1 to 7.
9. By the above addition, potassium is added as an alkali metal element.
   The method for manufacturing an optical fiber base material according to any one of claims 1 to 8.
10. By the above addition, rubidium is added as an alkali metal element.
    The method for manufacturing an optical fiber base material according to any one of claims 1 to 8.
11. It is made of silica-based glass and has a core containing alkali metal elements or alkaline earth metal elements.
    The concentration of the alkali metal element or the alkaline earth metal element in the core portion is different in a part in the longitudinal direction with respect to a part other than the part.
    Optical fiber base material.
12. It is made of silica-based glass and has a core containing alkali metal elements or alkaline earth metal elements.
    The core portion contains bromine in part or all of the longitudinal direction.
    Optical fiber base material.
13. The core portion contains chlorine and fluorine.
    The optical fiber base material according to claim 11 or 12.
14. The average concentration of chlorine in the core portion is 50 ppm or more and 2000 ppm or less.
    The optical fiber base material according to claim 13.
15. The average concentration of fluorine in the core portion is 2000 ppm or more and 3500 ppm or less.
    The optical fiber base material according to claim 13 or 14.
16. The average concentration of the alkali metal group in the core portion is 10 ppm or more and 100 ppm or less.
    The optical fiber base material according to any one of claims 11 to 15.
17. The core portion contains potassium as an alkali metal element.
    The optical fiber base material according to any one of claims 11 to 16.
18. The core portion contains rubidium as an alkali metal element.
    The optical fiber base material according to any one of claims 11 to 16.

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